Solid lubricants, such as molybdenum bisulfide, graphite and graphite fluoride, are only rarely applied in the dry powder state on the surfaces to be lubricated. More often they are used in the form of dispersions in oil, in organic solvents or in water, or even in the form of grease or paste. On numerous metallic pieces it is even more advantageous to deposit a lubricating varnish which adheres to metal.
It is known to form varnishes having a molybdenum bisulfide base and heat-hardening binders, such as corn syrups, silicones, asphalts and glycerine. In particular, such varnishes are described in the publications of NACA (National Advisory Committee for Aeronautics), Technical Note 2628 of February 1952 and Technical Note 2802 of October 1952.
The adherence of these varnishes to metallic surfaces depends on many factors such as the roughness of the metallic surface to be treated (which is a function of the method of burnishing, thus grinding gives roughnesses of between about 0.1 to 0.4 micrometers (.mu.m) while with sandblasting it is difficult to obtain a roughness below 1 .mu.m; the particle size of the solid lubricant which can vary from 10 .mu.m to less than 1 .mu.m, depending on the method of preparation and grinding used; and the nature of the metallic surface to be lubricated (for example, steel, brass, cast aluminum, tungsten carbide, zamak - a zinc alloy, etc.)
When one wants to prepare lubricating varnishes with a graphite fluoride base by relying on the teachings of the prior art, one encounters numerous difficulties. The varnishes, containing 2 parts by weight of glycerine and 1 part by weight of graphite fluoride (with a ratio of F/C=x=0.9) with an average 1 .mu.m particle size, adhere firmly, after heating at 250.degree. C. on steel pieces and on steel alloys of chrome and chrome-manganese. But on stainless steel there is less adherence. This is also true of metals other than steel. This bad adherence occurs on the one hand from the nature of the metal and, on the other hand, from the fact that the mixture of glycerine and graphite fluoride is relatively pastey and gives films that are too thick. If one tries to dilute this mixture by water to make it less viscous, one obtains a varnish which decants rapidly and which, consequently, is difficult to store. Moreover, the adherence on metals is very inadequate.
Aqueous dispersions of graphite fluoride can be prepared by grinding, for example, 20 parts by weight of 5 .mu.m particle size graphite fluoride in the presence of 70 parts by weight of water and 10 parts by weight of the sodium salt of a condensate of naphthalene sulfonic acid and formol (an aqueous dispersion of formaldehyde) marketed by PCUK Produits Chimiques Ugine Kuhlmann under the trademark DISPERGINE CB. The final particle size of graphite fluoride in the dispersion is advantageously approximately 1 .mu.m.
When such a dispersion is sprayed alone on ferrous metals and on aluminum, it gives, after heating at 250.degree. C., a uniform film which is not very lubricating and which is not resistant to the action of water. Dripping water on a steel plate coated with such a film causes rapid scaling and non-adherence of the film.
If glycerine is incorporated in the aqueous dispersion of graphite fluoride so that the graphite fluoride/glycerine ratio is equal to 0.4, and this composition is sprayed on aluminum or steel, a very irregular and barely adhering film is obtained after heating.
If, on the other hand, graphite fluoride powder, glycerine and an aqueous solution of DISPERGINE CB are mixed directly, compositions are obtained which, when applied on aluminum and dried at 250.degree. C., yield adhering varnishes, but which do not pass the dripping water test.